I had a wedding in October to go to and had the same dilemma - I ended up with a long wrap style cardy (you could get something really light in the sales with a similar effect) which I could just pull to over my bump and secure with a funky high belt, over a ... well, it was a stretch dress, but pretty much just a spaghetti strap, stretchy material to knee length (very, very basic and looks a bit like a nighty on its own!

) and you could even probably pick up something similar in most places. I figured that way I could wear everything again post baby ... just as well too because the blardy wedding was cancelled 2 weeks out! I'd have had kittens if I'd bought an actual maternity outfit!!